Transmission systems use a number of different transmission configurations and control schemes. The transmissions typically include a plurality of intermeshing gears that are either fixed to transmission shafts or that rotate freely on the shafts. Clutches associated with the freely rotating gears may be selectively engaged to establish a series of speed ratios between a prime mover output shaft and a transmission output shaft to transmit torque at a desired speed to drive a machine. Control systems for controlling the clutches typically respond to operator controls such as those directing the speed of the machine. The control system may send electrical signals to hydraulic valves that control the clutches. The control system thus causes the clutches to engage and disengage in predetermined combinations to accelerate, decelerate, and drive a machine as desired by the operator.
In a typical speed shifting operation, a first clutch (often referred to as an off-going clutch) is disengaged while a second clutch (often referred to as an on-coming clutch) is engaged. In doing so, the torque carried by the off-going clutch is transferred to the on-coming clutch to complete the shifting operation. To avoid or minimize interruptions in the transfer of torque from the input shaft to the output shaft during the shifting operation, the torque may be shared between the off-going clutch and the on-coming clutch as the shifting operation occurs. Due to the inertia of rotating components, the on-coming clutch must be capable of carrying a greater amount of torque during an upshift speed shifting operation as compared to that carried after the shift is completed. As a result, the on-coming clutch must be sized to accommodate the increase in torque that must be carried during the upshifting operation. However, since each of the clutches may be used as the on-coming clutch in some shifting operation, each of the clutches are typically configured to carry a larger load or amount of torque than they will likely carry after the shifting operation is completed.
In directional or shuttle shifting operations, a clutch may be used to slow a machine before changing directions. U.S. Pat. No. 4,989,470 discloses a system that permits controlled deceleration of a vehicle during directional shifts by disengaging the transmission from the engine through the disengagement of one of the clutches within the transmission. A different clutch is then engaged to create braking or tie-up within the transmission to create a load on the vehicle and slow the rotation of the transmission and the velocity of the vehicle. Once the vehicle has reached a desired velocity, typically close to zero, the clutch used for braking is disengaged and another clutch engaged for accelerating the vehicle in the desired direction.
The foregoing background discussion is intended solely to aid the reader. It is not intended to limit the innovations described herein nor to limit or expand the prior art discussed. Thus the foregoing discussion should not be taken to indicate that any particular element of a prior system is unsuitable for use with the innovations described herein, nor is it intended to indicate any element, including solving the motivating problem, to be essential in implementing the innovations described herein. The implementations and application of the innovations described herein are defined by the appended claims.